1. Field
This invention is directed generally to asphalt concrete performance testing equipment and in particular to those devices that attempt to predict the resistance of a designed asphalt concrete mixture to permanent plastic deformation, commonly referred to as rutting.
Asphalt concrete is the most common type of pavement surface material used in the United States. Due to the large investments most states have in asphalt pavements, it follows that they expend a significant amount of resources optimizing mix designs to achieve superior products. To facilitate the design process, different parts of the country design asphalt concretes to resist the types of failures that they most frequently encounter. Consequently, in warmer climates a major design consideration is the resistance a designed mix will have to rutting. It is cost effective to test designed mixes before they are placed to obtain information that will aid the engineer in predicting how they will perform after they are in service. Decades of research have yielded several pieces of equipment that test laboratory samples in the hope of predicting the tendency a designed mix will have to experience permanent plastic deformation while in service.
1. Description of the Prior Art
Equipment recently developed attempts to quantify the rutting susceptibility of asphalt concrete mixes. Loosely based on a European design, currently utilized laboratory equipment consists of an environmental chamber and a load beam. While a test specimen is environmentally conditioned to a temperature that approximates a maximum service condition, a loaded wheel rolls back and forth along a stationary beam. Each time the wheel passes over the sample, a load repetition is applied. As many loads are applied, the sample experiences permanent plastic deformation. Test results reflect the magnitude of the measured rut following a fixed number of load repetitions. The primary difference between the American and European devices is that the American device uses an inflated tube between the wheel and the sample to control contact pressure.
The equipment currently in use is limited in a number of ways. The most notable drawback is the applicability of test results. There is considerable debate regarding the performance similarities between a specially prepared laboratory sample and an in-place roadway pavement. Some feel that it is impossible to compact the small rectangular samples in a laboratory environment in such a way that they actually represent the mix as designed and constructed with respect to such parameters as confining stress, density and void ratio.
Critics of the current process note that simulations at temperatures that are representative of actual summertime service temperatures cannot be run without experiencing premature failures as evidence to support this position. Additionally, the special equipment that is required can make the process financially prohibitive.
Further, excessive time may be required to conduct the test as the load wheel must change direction between each application. For samples that are designed to be rut resistant, a successful proof test may take many hours to complete. This limitation would hinder the use of such devices for field quality control, where additional test time may mean that a significant amount of inferior pavement has been placed.
The above mentioned equipment, while providing a method for approximating the rutting susceptibility of an asphalt, leaves substantial room for improvement, For examples a machine that could utilize the standard design specimens (see AASHTO T 245) would avoid the question of similarity between rutted samples and constructed pavements. Also, the problem of confining stress could be completely avoided if there were a way to test the specimen without extracting it from its compaction mold. Consequently, existing compaction equipment could be utilized to prepare the test specimens, thereby avoiding the cost of new compaction equipment entirely, Further, use of existing environmental conditioning equipment would make a rut testing device more attractive for industry-wide use. Due to the size of today's standard samples and existing equipment, however, a radically different approach to the method of applying the load repetition has to be considered.